Clarify colormap_name requirements and usage

[emmanuelmathot]

Update the CHANGELOG and README to specify that colormap_name must be a standard matplotlib colormap name, ensuring a consistent naming convention for raster bands. Include additional details on how to use colormaps with titiler.

fixes #14

This is a breaking change and will require a version bump

[vincentsarago]

must be a standard matplotlib colormap name,

We also have custom colormap names in titiler

I'm starting to feel that it will be difficult to generalize renders for many backends/clients 🤷

[m-mohr]

Such an extension will be a compromise if we don't want this to be the "titiler" extension (which should then be the name), and never be supporting everything that a specific implementation might support. User could use colormap instead of pre-defined names for those additional cases. Coming up with a more interoperable compromise render extension is generally a good way forward IMHO. That could make it work for the "80%" of the cases for everyone instead of 100% only for titiler.

[m-mohr]

Generally looks good to me. Should we extract the allowed names to an enum in the schema?